When there is no sunlight, there are 2 options:
a. Because the boiler is insulated with a 5cm thick polyurethane insulation, the sanitary water inside the boiler stays hot for several hours, and as such you can continue using it even at night or on a cloudy day. This thermal inertia depends on many factors such as the initial temperature of the water in the boiler; outside air temperature; usage. For this reason, it is best to install a larger boiler when possible in order to use the water mass to retain more heat during non-sunny periods;
b. Inside the boiler there is back-up electrical resistance from the thermostat that heats or keeps the water hot when the sun is unable to heat the water.